OCT is a non-invasive imaging modality that provides micrometer scale resolution of tissue structures over depth ranges of a few millimeters. The technique has found a number of biomedical applications, most notably in ophthalmic and cardiovascular imaging.
Swept-source OCT (SSOCT) is an improvement to OCT that provides a dramatic sensitivity advantage over traditional time domain techniques. However, SSOCT suffers from an inherent (i.e. sample independent) reduced imaging depth range, typically limited to between 1 and 4 mm. Optical attenuation from absorption and scattering in tissue typically limit how much light is recovered from depths beyond a few millimeters, and thus for many applications this inherent reduced depth range is not the limiting factor in determining the practical imaging depth. However, several important OCT applications would benefit from extended imaging depths, including ophthalmic imaging of the anterior segment, small animal imaging, endoscopic imaging, and catheter imaging of coronary arteries.
Extending the imaging range of SSOCT has thus been an area of interest for which a number of techniques have been developed. However, all of these techniques are accompanied by drawbacks including reduced sensitivity, reduced axial resolution, reduced imaging speed, required lateral oversampling, increased system complexity, increased cost and/or increased signal processing overhead. In addition, many of these techniques produce incomplete suppression of the complex conjugate artifact, resulting in distracting “ghost” images.
SSOCT suffers from a limited inherent imaging depth range due to two factors. The first of these stems from the fact that SSOCT extracts depth information from the Fourier transform of a spectral interferogram. As the spectral interferogram can only be recorded as a real signal, its Fourier transform is necessarily Hermitian symmetric. Consequently, positive and negative displacements from the zero pathlength difference position (DC) cannot be unambiguously resolved, giving rise to mirror image artifacts. This phenomenon is termed the complex conjugate ambiguity. These artifacts can be avoided by placing the zero pathlength difference position outside of the sample, which results in two mirror images of the sample being acquired in the positive and negative frequencies. While this technique resolves the ambiguity between positive and negative displacements, it also effectively halves the useful imaging range of SSOCT systems, and may result in the appearance of “wrapped” mirror image artifacts if the sample moves unexpectedly.
The complex conjugate ambiguity would not pose such a problem if it were not for the fact that the total imaging range is also limited by a phenomenon known as sensitivity fall-off. The instantaneous linewidth of the swept laser in SSOCT systems (and the spectral bandwidth of each spectrometer pixel, in SDOCT systems) can be thought of as a sampling function that interrogates the intrinsic spectral interferogram. The spectral interferogram is sampled by, and thus convolved with, the laser linewidth (or spectrometer pixel bandwidth), which results in reduced fringe visibility when the fringe period is small. As smaller fringe periods (i.e. higher fringe frequencies) correspond to deeper imaging depths, this reduced visibility results in decreasing sensitivity with increasing imaging depth.
An effective method for resolving the complex conjugate ambiguity is heterodyne SSOCT (HSSOCT), which resolves the ambiguity by shifting the peak sensitivity position image away from DC, such that positive and negative displacements from that position can be discerned. As this technique shifts, rather than suppresses, the complex conjugate signal, it completely resolves the artifact. In addition, HSSOCT does not result in any reduction in imaging speed or require lateral oversampling. In this method, two frequency shifters, usually acousto-optic modulators (AOM's) (though electro-optic modulators (EOM's) have been used) are used to apply different modulation frequencies to the sample and reference arm. While effective, this technique is limited in that the modulators are expensive and difficult to implement. More significantly, AOM's tend to have appreciable insertion losses, resulting in reduced sensitivity, and also often restrict optical bandwidth, resulting in reduced axial resolution. In addition, processing of the acquired data requires either hardware demodulation or significant additional post-processing steps.